CAPE

Underscores the importance of a sustained -AO/NAO in setting us up for the perfect storm track, with easy access to fresh cold air. The rest was due to the pronounced STJ, courtesy of the moderate Nino. Memories...

Yup. It was rare and perfect. Sure it was a slushy, foggy mess by Xmas day and it rained, but that's how we roll here.

Hard to beat the December 2009 storm for me. December is mostly a Fall month around here, and we rarely see a KU event before Xmas.

We always talk about the importance of HL blocking, but many times it is not sustained. In 2009-10 it was pretty much wall to wall, legit -AO/NAO for the entire winter (see my display pic). That sets us up so we don't have to thread the needle. Ofc it was also a Modoki Nino. We get that combo and its hard not to see a few big winter storms.

Getting it to snow here is fluky most winters. It's probably become more so recently.

Capital Weather Gang article with a potpourri of winter outlooks. DT is thinking back-loaded and above normal snowfall for DC. https://www.washingtonpost.com/weather/2020/10/21/washington-dc-winter-forecasts-2020/?fbclid=IwAR0KbqBko3L1eyFPIZbWE5vr9l-LDSN2NUloXhSlZxFWXhsDGyG7lWz-j4w

We generally need some HL 'help' to get average or above average snow. In the absence of a -AO/NAO, a -EPO can get it done.

There is a correlation between the AO phase and the ENSO state, and the AO is the index most highly correlated to snowfall for DC-BWI. The NAO is almost always in the negative phase when there is a negative AO, so the NAO is a factor. I don't have the numbers, but I would bet a predominately -AO for DJF is more likely during a Nino than a Nina. Not sure if there is a large enough sample size, but a Moderate, CP-based Nino might have the highest correlation to a -AO. @psuhoffman can probably give the numbers off the top of his head.

It will be interesting to see if the extended range GEFS is on to something, as it continues to advertise increasing h5 heights up top, and especially in the NAO domain towards mid November. Also keeps the Pacific (puke) in check. Some of the MJO forecasts have the tropical convection moving into the W Pac, and then possibly into Phase 8, so that may give some credibility to the idea of increasing high latitude h5 heights down the line.

Good ol' CFS at range.. December would be acceptable too, with a +PNA, and the -EPO taking shape. Not the likeliest of outcomes in a moderate Nina.

Just my opinion, but I think it will take some -EPO episodes to save us from another generally crap winter. I don't see any significant indicators suggestive of a predominant negative AO/NAO, other than possibly solar min, and that correlation is nebulous at best.

lol It's probably neutral, not that it matters at hour 816 on a GEFS run.

^That little blue spot off the MA coast is probably a blizzard.

Since we can, why not? The super duper extended GEFS looks blocky for much of November.

Surprise!

The NAO is notoriously unpredictable at long leads. One thing we have to go on is persistence. The NAO has been predominately positive during winter for many years.

Go back and read the previous few pages ITT. This has been discussed at length. Also I am not sure anyone is 'canceling' winter. Well maybe Ji has, but he doesn't count.

It will snow this winter. Projected mean patterns are just that. We normally get a few shots each winter. Last winter was an o-fer for most, and while it could happen again this winter, it probably won't. Nina winters don't often produce above avg snow, and can be super frustrating with the typical NS dominant flow/late developing coastals. I will say this again- the last 2 Ninas produced slightly below median, and slightly above avg snowfall here, respectively. Both produced big snowstorms(one was a blizzard) for the immediate coastal plain, with no blocking. Clearly better outcomes than the so called Nino and warm neutral of the past 2 winters for my yard. And yes, while DC-BWI and west missed both of those Nina events, it wouldn't have taken much for better outcomes in those areas. Ofc this is a imby business, so I don't care.

Significant HL blocking can mitigate a lot of bad elsewhere. Unfortunately we are on a bad run in that dept. I suppose we are due.

Looking at the latest LR global means, some significant changes up top. Lots of blue, and CFS weeklies continue that through November. The -NAO period was possibly just a transitional thing. The CFS run of indicating HL help for the winter months seems to have finally ended. Its h5 depiction for DJF aligns with most other climate/seasonals now. Has a +AO/NAO throughout. The mean EPAC ridge location/orientation could be worse though. Actually has hints of an EPO ridge, esp for late winter. fwiw.

It likes to do this in the LR.

Boilerplate. I guess it's as good as anything else for a general winter outlook.

There are other factors/indices that influence snowfall here in a given winter, as you well know.

Unless it is a legit, sustained west-based block- but that is unlikely to occur unless the AO is also negative. I guess there is some possibility the mean Pac ridge is close enough to the west coast in early winter to pop a +PNA for a time. Some of the climate models have suggested this before the Pac goes to total crap by mid winter. Latest CFS runs have backed off on that idea, and also any semblance of HL help. It is starting to get a clue.

This is true but one thing to consider here is the official method for measuring the AO and NAO phases(surface based), vs the sensible weather impacts we associate with the h5 heights in those domains. They mostly should be aligned, but not always.